With the increase in the development and use of mobile devices, electric vehicles, hybrid vehicles, power storage apparatuses, uninterruptible power supplies, and the like, the importance of a secondary battery as a main power source is growing. Accordingly, much research on a secondary battery satisfying various demands thereof has been carried out.
Generally, prismatic or pouch-shaped secondary batteries is heavily demanded in terms of shape, and in terms of material, lithium-based secondary batteries are mainly used due to their high energy density and large discharging capacity per unit time.
In the research of such a secondary battery, enhancing the safety thereof is an important aspect. Lithium secondary batteries may be heated due to internal short-circuit, overcharging and overdischarging, which can lead to the decomposition of an electrolyte and thermal running, causing the pressure inside the batteries to rapidly increase, thereby resulting in the batteries to inevitably explode.
The internal short-circuit causing the explosion of the secondary battery is generated by various reasons including the fall of the secondary battery and external impact applied therein. Conventionally, the secondary battery consists of at least one unit cell, a battery management unit (BMU) and a pack housing surrounding them. Each unit cell and the BMU are connected through an electrode lead. In the electrode lead, a plurality of electrode tabs connected to each unit cell are welded. The electrode tabs extend from a thin aluminum or copper plate or is attached thereto, and exhibit very low mechanical strength due to the inherent characteristic thereof.
Meanwhile, if the secondary battery falls or is subject to external impact, the unit cells sealed in the pack housing may move. In this case, impact energy may focus on a connection part between the unit cell and the BMU. The impact energy may accumulate in a connection part having a poor mechanical strength among many connection parts of the secondary battery and cause the electrode tab to burst. The burst of the electrode tab may generate internal short-circuit to cause a safety accident such as the explosion of the secondary battery. Accordingly, it is necessary to detect external impact to protect the secondary battery.
For this, Korean Patent No. 10-0739080 discloses an acceleration sensor equipped in a vehicle to detect external impact applied to the vehicle. In this patent, when the acceleration sensor detects an impact level higher than a critical value, large impact is considered to be applied to a secondary battery, which stops the use of the secondary battery.
However, since this prior art considers only the variation of acceleration in a vehicle, there is a limit in controlling the use of a secondary battery or the stop thereof. For example, if an impact level above a critical value is detected by an acceleration sensor but the impact is applied to the secondary battery in a level not causing the short-circuit thereof, or on the contrary, if an impact level below a critical value is detected by an acceleration sensor but the impact is greatly applied to the secondary battery in a level causing the short-circuit thereof, it is difficult to properly protect the secondary battery. Accordingly, a system for detecting impact applied to the secondary battery only depending on the variation of acceleration has low accuracy and therefore fails to sufficiently protect the secondary battery from external impact.